A nearby galaxy cluster is facing an intergalactic headwind as it is
pulled by an underlying superstructure of dark matter, according to new
evidence from NASA's Chandra X-ray Observatory. Astronomers think that
most of the matter in the universe is concentrated in long large
filaments of dark matter and that galaxy clusters are formed where these
filaments intersect.

A Chandra survey of the Fornax galaxy cluster revealed a vast,
swept-back cloud of hot gas near the center of the cluster. This
geometry indicates that the hot gas cloud, which is several hundred
thousand light years in length, is moving rapidly through a larger, less
dense cloud of gas. The motion of the core gas cloud, together with
optical observations of a group of galaxies racing inward on a collision
course with it, suggests that an unseen, large structure is collapsing
and drawing everything toward a common center of gravity.

"At a relatively nearby distance of about 60 million light years, the
Fornax cluster represents a crucial laboratory for studying the
interplay of galaxies, hot gas and dark matter as the cluster evolves."
said Caleb Scharf of Columbia University in New York, NY, lead author of
a paper describing the Chandra survey that was presented at an American
Astronomical Society meeting in New Orleans, LA. "What we are seeing
could be associated directly with the intergalactic gas surrounding a
very large scale structure that stretches over millions of light years."

The infalling galaxy group, whose motion was detected by Michael
Drinkwater of the University of Melbourne in Australia, and colleagues,
is about 3 million light years from the cluster core, so a collision
with the core will not occur for a few billion years. Insight as to how
this collision will look is provided by the elliptical galaxy NGC 1404
that is plunging into the core of the cluster for the first time. As
discussed by Scharf and another group led by Marie Machacek of the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., the hot
gas cloud surrounding this galaxy has a sharp leading edge and a
trailing tail of gas being stripped from the galaxy.

"One thing that makes what we see in Fornax rather compelling is that it
looks a lot like some of the latest computer simulations," added Scharf.
"The Fornax picture, with infalling galaxies, and the swept back
geometry of the cluster gas - seen only with the Chandra resolution and
the proximity of Fornax - is one of the best matches to date with these
high-resolution simulations."

Over the course of hundreds of millions of years, NGC 1404's orbit will
take it through the cluster core several times, most of the gas it
contains will be stripped away, and the formation of new stars will
cease. In contrast, galaxies that remain outside the core will retain
their gas, and new stars can continue to form. Indeed, Scharf and
colleagues found that galaxies located in regions outside the core were
more likely to show X-ray activity which could be associated with active
star formation.

The wide-field and deep X-ray view around Fornax was obtained through
ten Chandra pointings, each lasting about 14 hours. Other members of
the research team were David Zurek of the American Museum of Natural
History, New York, NY, and Martin Bureau, a Hubble Fellow currently at
Columbia.